Hitherto there have been proposed several methods of manufacturing a container from an expanded thermoplastic resin sheet. For example, Japanese Patent Publication No. 44943/1974 discloses a metod of fabricating a container from a plain thermoplastic resin sheet which has been previously cut into an extended shape with side walls surrounding a bottom area, with this being accomplished by pressing a heated V-edged parallel cross shaped device to form the bend lines, followed by bending the sheet upward along these bend lines to form the side walls and building up a container. The said process has a drawback in requiring extra manipulative steps because of the use of prefabricated material made by cutting and removing the marginal pieces and forming the bend lines into an unfolded sheet.
Japanese Patent Publication No. 2181/1975 discloses a method of manufacturing a container from a plain thermoplastic resin sheet cut into a fixed size, by pressing a heating and cutting means provided with parallel cross shaped V-edged pressing blades and a marginal cutting blades thereto to provide the bend lines and the side wall joining portions, and forming the side walls by bending upward along the bend lines. The said method, not provided with a cutout piece ejecting mechanism, is susceptable to production of defective products because, if the marginal portions which should be removed remain in a fused state even in small portions, said marginal portion may be inserted simultaneously with the expanded sheet into the concave portion of the female mold at the time of the insertion. In both of the above methods, the cross section of the edge of the linear pressing blade has an angle of about 90 degrees.
Japanese Patent Publication Nos. 6874/1973 and 53876/1973 also disclose box making machines having special blades, but neither of them is equipped with a cutout piece ejecting mechanism nor pays any special consideration to the edge angle of the V-shaped hot blades.
West German, Laid-open Pat. No. 1,218,139 discloses a method of manufacturing an illumination case from a thermoplastic synthetic resin sheet by providing the cutting lines over all surfaces thereon, applying heat thereto along the bend lines, followed by bending, wherein the V-edged V-groove is cut on the bend line, then the linear electric heating element is guided along the V-groove in parallel with the groove in a condition such as to prevent contact after which the sheet is bent along the V-groove. It must be noted, however, that this method uses a non-foaming synthetic resin sheet, such as an acryl resin, as a material, wherein, for example, in bending the sheet to an angle of 90.degree., the groove must be formed by scraping off such as by milling, etc. to a cross section of 90.degree. in advance. The present invention offers a method of manufacturing a container which is an improvement over prior procedures and does not, for example, necessitate previous scraping steps to form the grooves.
Also, while there are, among the known containers, only boxes having a square bottom and upright side walls, the present inventor has found it possible, as a consequence of various studies, to readily produce containers having side walls at an inclination of 45.degree. or more from the bottom surface.